As a linear motor, a cylindrical one as shown in FIGS. 32A and 32B is known. FIG. 32A is a sectional view of the linear motor taken along its axial direction, and FIG. 32B is a sectional view of the linear motor taken along a direction perpendicular to its axis. In the linear motor shown in FIGS. 32A and 32B, an annular multilayered yoke (iron core) 1202 formed by combining arcuated multilayered yoke members 1202a each shown in FIG. 32C is formed outside a cylindrical support rod 1201. A plurality of annular partial coils 1203 are arrayed outside the multilayered yoke 1202 along the axis of the cylindrical support rod 1201. The cylindrical support rod 1201, annular multilayered yoke 1202, and plurality of annular partial coils 1203 described above form the stator of the linear motor.
The movable element of the linear motor is formed outside the annular partial coils 1203 of the stator, and is formed of an annular yoke 1205 and a plurality of annular partial magnets 1204. The plurality of annular partial magnets 1204 include those magnetized in the radial direction of the annulus and those magnetized in the axial direction. More specifically, the direction of magnetization of each annular partial magnet 1204 is determined such that an alternating field is generated inside a cylinder formed of the plurality of annular partial magnets 1204.
In this conventional linear motor, a so-called Halbach layout generates a field close to a sine-wave field of two periods in the cylinder. The annular yoke 1205 is arranged outside the plurality of annular partial magnets 1204, and serves as a so-called back yoke. In other words, the annular yoke 1205 is formed on the rear side of the annular partial magnets 1204 so as to increase the magnetic flux of the magnets. In this example, since the annular partial magnets 1204 form the Halbach layout, the back yoke can be thin.
The annular multilayered iron core 1202 of the stator serves to intensity the magnetic field of the plurality of annular partial magnets 1204 generated inside the annulus. In this example, since the annular partial magnets 1204 form the Halbach layout, the annular multilayered yoke 1202 must be thicker than the back yoke.
The annular yoke 1205 as the back yoke can be a plain one as it is integral with the magnets 1204. As the annular multilayered yoke 1202 of the stator moves relative to the magnets 1204, it has a multilayered structure with an insulating layer formed in a direction along its axis, thereby preventing an eddy current.
The plurality of annular coils 1203 are formed of a plurality of phases (two phases A and B in this example).
This linear motor is driven by general sine-wave driving, and is controlled such that the current and magnetic flux intersect each other. Note that this arrangement employs a movable magnet, stationary coil method, and requires coil switching in addition to general sine-wave driving. This is to supply power to, of the plurality of partial coils 1203, only those that face the plurality of partial magnets 1204, so heat generation is reduced. FIG. 33 shows the switching timing for the phase A. In the state shown in FIG. 33, when the movable element moves to the right, the phase A becomes OFF, and the phase A′ becomes ON. Conversely, when the movable element moves to the left, the phase A′ becomes OFF and the phase A becomes ON. Regarding phases that oppose other magnets, the phases A and B are all OFF.
The conventional cylindrical linear motor described above has the following problems in terms of the manufacture and performance, since the partial magnets 1204, the partial coils 1203, and the multilayered yoke 1202 of the stator are cylindrical.
The first problem is that the multilayered iron core of the stator is difficult to fabricate, and an eddy current is difficult to prevent. Conventionally, the annular multilayered yoke 1202 is formed by combining the arcuated multilayered yoke members 1202a. It is difficult to fabricate such arcuated multilayered yoke members 1202a. Hence, rectangular parallelepiped multilayered yoke members must be fabricated first, and then must be formed into arcuated shapes by wire cutting or the like. With this method, a large number of processing steps are required. It is thus difficult to fabricate a structure with a length corresponding to the length of the stator in the axial direction with one process. Also, it is difficult to obtain highly precise arcs.
The magnetic fluxes of the annular partial magnets 1204 enter the multilayered yoke 1202. Hence, in the concentric arrangement shown in FIGS. 32A and 32B, it is desirable that the multilayered structure of the multilayered yoke 1202 is ideally formed completely radially. In the arrangement of FIGS. 32A and 32B, however, since the layers of the multilayered yoke 1202 are not completely radial, an eddy current is undesirably generated by some components of the magnetic fluxes. It is still also difficult to fabricate a completely radial multilayered iron core.
The second problem is that the magnet unit of the movable element is difficult to fabricate and variations in thrust are caused. The magnet unit of the movable element is formed by inserting the plurality of annular partial magnets 1204 inside the cylindrical yoke 1205. When inserting the annular partial magnets 1204 in the cylindrical yoke 1205, it is difficult to set their tolerances. When the tolerances are decreased, the positional precision may improve. With this structure, however, as the short annular partial magnets 1204 must be inserted in the long cylindrical yoke 1205, scuffing tends to occur, and it is difficult to insert the cylindrical magnets 1204 deep into the annular yoke 1205. Conversely, when the tolerances are increased, the annular partial magnets 1204 may be inserted in the cylindrical yoke 1205 easily. However, tilt or eccentricity may occur so the annular partial magnets 1204 cannot be attached with high precision. As the cylindrical yoke 1205 and each annular partial coil 1203 come into contact with each other through only one point, they are not fixed securely to each other. Furthermore, magnets and iron attract each other, and magnets attract and repel each other, making the assembly more difficult. When the assembly precision of the cylindrical magnets 1204 is degraded, variations in thrust are caused, leading to degradation in precision of the linear motor. A method of making the constituent members of the cylindrical yoke 1205 and annular partial magnets 1204 into small pieces and assembling them has been studied. With this method, however, a cylindrical surface must still be fixed to another cylindrical surface. The number of components increases to increase the number of processing steps, and to guarantee the assembly precision as a whole becomes more difficult. In this manner, a thorough solution cannot be made.
The third problem is that it is difficult to draw out a conductor wire from the stator coil, thus decreasing the thrust. In the annular coil 1203, a conductor wire at the winding start portion is always on the inner side. To draw out the conductor wire at the winding start portion to the outside of the annular coil 1203, the conductor wire need be drawn out to the outer surface of the annular coil 1203, and need be drawn out along the outer surface of the annular coil 1203 in the axial direction. Also, a space such as a groove need be formed in the annular multilayered yoke 1202, and the conductor wire need be extended in the space. In the former case, the lead conductor wire is extended in the gap between the magnets 1204 and coils 1203. When the mechanical clearance between the magnets and coils (minimum distance between the magnets and coils) is to be made constant, the magnetic gap between the magnets 1204 and stator multilayered iron yoke 1202 must be increased. This decreases the thrust. In the latter case, machining of the multilayered iron core 1202 becomes more difficult, and the magnetic gap increases partially, leading to a decrease in thrust.
The conductor wire at the winding end portion of each coil 1203 is always located on the outer surface of the coil. When this conductor wire is directly extended along the outer surface of the coil 1203, a decrease in thrust is caused to accompany an increase in magnetic gap. If the conductor wire at the winding end portion is guided to the inner side of the coil 1203 first and is then extended on the inner surface of the coil 1203, the same problem as that arising when the conductor wire at the winding start portion is extended on the inner surface of the coil 1203 occurs. That is, the multilayered iron core becomes difficult to machine, and the thrust is decreased by an increase in partial magnetic gap.